Automatic transmissions manufactured for conventional automobiles are not well suited for racing vehicles. A number of problems, including the components being subjected to a higher rotational speeds as well as being subjected to higher stress, arise when automatic transmissions manufactured for conventional automobiles are used in racing vehicles. This can cause breakage of a large rotating component into fragments that can cause breakage of other parts of the transmission. A number of prior art patents, including U.S. Pat. Nos. 5,193,415 and 5,090,528, disclose solutions to this problem.
One of the component parts of an automatic transmission is a part called the valve body. The valve body consists of passageways and valves that shuttle oil (automatic transmission fluid) around to various sections of the transmission. When the oil is commanded by the valve body to go to a certain section of the transmission, that section of the transmission performs its intended function. The functions could include to shift up, down, softly, reverse, etc. The valve body gets its input from a variety of sources which are typically the driver through the shift lever, the engine speed, the load and transmission oil temperature. The valve body is designed or programmed to control the transmission according to a set of rules which relate to all of the inputs together.
One of the commands that the valve body issues is how hard the transmission should shift gears. This is done by including in the valve body a spool valve that is moved by a vacuum canister (also called a "vacuum modulator") that is attached to the outside of the transmission case. The vacuum canister reacts to the amount of vacuum present in the intake manifold of the engine. At light loads, there is a high vacuum present. At heavy loads, i.e. full throttle, there is very little vacuum present.
At heavy loads, it is desirable to make gear shifts as quickly and abruptly as possible. This maintains the maximum amount of power flowing through the drive train and it prevents the destructive slipping of the clutches which might occur due to the heavy loads during a shift. At light loads, the vacuum modulator moves the spool valve in the opposite direction which tells the transmission to make a soft or lazy shift. The power going to the transmission is low so that clutch slippage is minimal. The purpose of the soft shift is to prevent an uncomfortable "bone jarring" gear change from occurring at light loads.
In drag racing, all shifts are made at full power and the quickest shift possible is desired so that a modulator circuit is not required. The modulator circuit is not wasted, however, because the transmission is modified by adding a transmission "brake". This brake usually consists of a valve body that has had its internal oil passageways changed (specifically the modulator circuits). The vacuum modulator canister that was attached to the outside of the transmission case is replaced with an electric solenoid. The solenoid pushes on the former modulator spool valve. Instead of sending oil to tell the transmission how hard to shift, the modulator spool valve sends oil to the reverse gear apply piston. When the oil is directed to this piston, it moves and locks up the reverse gear clutch which applies reverse gear.
With the reverse gear applied, the transmission is placed in a low forward gear as well. Thus, the transmission is jammed because it is in reverse and forward gear at the same time, effectively creating a transmission "brake". The engine can run at full throttle and the car won't move because of the jammed transmission.
During drag racing, two cars line up side by side on the starting line. After a countdown, typically accomplished by a Christmas tree sequentially counting down a series of lights, the driver releases an electrical switch that supplies power to the transmission brake solenoid, which has replaced the vacuum modulator canister, resulting in the release of the reverse gear. Since the car is also in low gear, the full power of the energy is instantly applied to the rear wheels of the car resulting in maximum acceleration.
For absolute maximum acceleration with existing transmissions, release of the rear gear must be made as quickly as possible. This is desirable to "shock" the rear tires and suspension to make the tires adhere to the pavement instead of spinning. In addition, it is desirable to minimize the amount of reaction time for the car to start moving from the time the electrical signal is removed from the transmission brake solenoid. This permits the racing vehicle to have a quicker start time.
Typical conventional automatic transmissions have passageways that feed oil through the housing and into the piston of the reverse gear. This feed oil passageway supplies the oil to the reverse gear piston which locks up the reverse gear clutches and applies reverse gear. At the same time, this passageway must be the passageway through which the oil is released, flowing in an opposite direction to release the reverse gear. In the prior art, in an effort to speed up the oil flow out of the piston area when the transmission brake is released, racers have drilled out the feed passageway to increase the size of the passageway. Although the larger passageway allows oil to flow faster out of the area, there is a trade off because the larger passageway increases the total amount of oil in the reverse gear apply system. That is, when the gear is first applied, more oil must flow into the reverse gear apply system to fill the enlarged passageway. Therefore, more volume must consequently be removed from the reverse gear apply system when the reverse gear is released. Thus, this trade off limits the amount of improvement possible in decreasing the starting time of a racing vehicle. There is a need to speed the oil flow out of the piston area without drastically increasing the amount of oil in the reverse gear apply system.
By designing a new transmission case, it is possible to create more efficient fluid passageways that release the transmission "brake" extremely quickly. For some car designs, however, a "brake" release that is too fast causes the tires to be "shocked" too hard, resulting a loss of traction (a common condition in stick shift racing cars). An adjustable means to slow down the release of the transmission "brake" is needed to make the transmission compatible with different racing vehicles.
Another problem hindering the speed of release of the transmission brake is the amount of air present in the reverse gear apply piston area. Under racing conditions, air is always present in the oil because of foaming. Foaming occurs when oil is churned up by the rotating parts while in the presence of air. A quick decrease of the volume of oil in a confined reservoir without air present results in a quick decrease of pressure in the confined reservoir. If air is present, the air expands thus slowing the quick decrease of pressure, which in turn slows the release of the transmission brake. Further, since the amount of air in the system is never the same, the release times of the transmission brake will vary. There is a need to bleed the air consistently in such a system as successful drag racing today requires that a vehicle repeat within thousandths of a second.
Existing racing transmissions sometimes have a hole drilled in the backside of the reverse gear apply piston to bleed air out. This solution has three drawbacks. First, the piston must be installed with the hole on the top and it must not be allowed to rotate with time (since the air will always rise to the top-above the fluid). Second, the hole is on the clutch disc side of the piston where dirty, unfiltered oil is present. This can allow chunks of debris or clutch dust to plug the hole, and it allows dirty oil into the pressure side of the piston causing permanent scoring of the seal area and damage to the piston seals. Finally, the clutch plates contact the outer diameter face of the piston. In order to prevent the clutch plates from blocking the bleed hole, the hole must be drilled near the center of the piston, thereby leaving a large air pocket at the top portion of the piston.
Another problem with such a transmission is that under racing conditions, the engine flex plate sometimes fails and shatters, sending flying debris out of the transmission/flywheel area similar to an explosion. The racing sanctioning bodies have mandated the use of "explosion proof" flexplates. For extra safety, "explosion shields" are also mandated that attach to the transmission in the flywheel area to prevent parts from flying up through the floorboards of a car. There are currently no requirements to prevent debris from exiting the bottom of the transmission/flywheel area. The existing shields are straps of steel or composite materials that bolt to the top of the transmissions in the flexplate area. The existing production transmissions are not designed for, nor are adequate to act as explosion shields. During an explosion, the entire bellhousing (the flexplate area of the transmission housing) shatters and is destroyed. There is a need for a shielding means to protect the bellhousing, engine compartment, car interior, and roadway from flying debris in the case of a flex plate failure.
One current solution is to separately cast the bellhousing so that it is strong enough to resist flexplate explosions. The drawback to this solution is that the entire front of the transmission must be cut off and machined to accept the separately cast bellhousing unit. There is a need for a convenient and lightweight flexplate shield.